feat(examples): add comprehensive API examples

[lightcap]

Summary

Adds comprehensive API usage examples matching the 1Password Python SDK's example coverage:

• examples/example.rs — Full SDK surface demo: secrets (single, batch, TOTP), password generation (PIN, memorable, random), item CRUD with fields/sections/tags/websites, batch operations, item sharing, SSH key items, document items, file field attachments, and vault lifecycle management
• examples/desktop_app.rs — Expanded to include vault CRUD, batch item operations, and group permission management (grant, update, revoke)
• examples/README.md — Usage guide with environment variable setup and operations coverage matrix

Note: This PR is draft because the examples have not yet been tested against a live 1Password account. We're waiting for access to a service account to verify runtime behavior.

Test plan

• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo test passes (23 tests)
• cargo check --example example --example service_account --example desktop_app --features desktop compiles
• Run example.rs against a live 1Password service account
• Run desktop_app.rs against the 1Password desktop app

🤖 Generated with Claude Code

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Note

Medium Risk
Mostly adds/expands example code and documentation, but also changes ClientConfig.system_os serialization (mapping macos to darwin), which could affect core/API behavior on macOS if consumers rely on the previous value.

Overview
Adds a new examples/example.rs that exercises most of the Rust SDK surface area (secrets, password generation, item CRUD + batch ops, sharing, SSH keys, documents, file attachments, and vault lifecycle) and introduces an examples/README.md with setup instructions and an operations coverage matrix.

Expands examples/desktop_app.rs from a single secret lookup to a broader desktop-auth demo including vault CRUD, item listing, batch item create/get/delete, and group vault permission grant/update/revoke (with best-effort cleanup). examples/service_account.rs is updated to accept OP_SECRET_REF and avoids printing secret contents.

In core, ClientConfig.system_os now normalizes macos to darwin via normalize_os() and adds tests to lock in the mapping.

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